An apparatus for extracorporeal blood treatment, comprising: - a blood withdrawal line (3) having an end intended to be connected to a cardiovascular system of a subject； - at least one blood treatment unit (6) comprising a semipermeable membrane (10) that divides the unit itself into a first and a second chamber (11, 12), the first chamber being connected to a second end of the line blood withdrawal (3)； - a return line (7) of the treated blood to the patient having a first end connected to the first chamber (11) of the blood treatment unit (6) and a second end intended to connect to the patient's cardiovascular system； - at least one first pump (13) that works in correspondence of the blood withdrawal line； - at least one second pump (14) that works in correspondence of the blood return line to the patient； - a discharge line (15) connected to the second chamber (12) of said treatment unit (6) to receive residual fluid at the outlet of the unit itself； - at least one first blood reservoir (21) operating on the blood return line (7) upstream of the second blood pump (14)； - at least one detector (22) predisposed to detect at least one parameter selected from a group comprising: - a pressure in said first reservoir, - a level of blood present in said first reservoir, - a volume of blood present in said first reservoir tank, - a pressure in a line of the return line upstream of the second pump, and predisposed to emit a signal corresponding to a measured value of the parameter； - a control unit (100) connected to the first pump, the second pump and said detector, wherein said control unit is configured to operate in a first mode of operation, corresponding to a single access configuration of the extracorporeal circuit , and in a second mode of operation, corresponding to a double access configuration of the extracorporeal circuit, the control unit being configured, in said second mode of operation, to perform the following steps: - receiving the signal emitted by sai
